FIG. 3.

Reporter constructs and promoter activity for P_RM⁺ and prm240. (A) Structure of reporter constructs (not to scale). The three constructs shown (see the supplemental material) encode an mRNA that is terminated at T_imm; hence, all three mRNAs are identical and their levels of expression are directly comparable (1). A second version of each construct contained the r1 allele in O_R3, which weakens CI binding (see text). Maps are not to scale. (B and C) Reporter assays. Cells were grown at the temperature and in the medium indicated in the figure. Two reporter constructs were used, one with P_RM⁺ and the other with prm240 (see text and panel A, middle construct). For each construct and growth condition, two strains were used, one lacking CI and one carrying a plasmid with a weak lacP::cI fusion. The latter strain was grown with no IPTG or with the indicated levels of IPTG, providing various levels of CI. Data for P_RM⁺ at 30°C were obtained in separate experiments and are shown in Fig. S3 in the supplemental material. (B). Expression of P_RM⁺ and prm240 at 37°C. (C). Data for prm240 fusions, with an expanded scale. As judged by a gel shift assay (not shown), CI levels in the reporter strain grown in M9 medium at 30°C and in LB medium at 37°C with 2 mM IPTG were roughly 3 times and 1 time, respectively, the level in JL5932 grown in parallel; in LB medium at 37°C, the CI levels in cells grown at 0.01 and 0.2 mM IPTG were roughly 0.1 and 0.5, respectively, of the lysogen level, bracketing the value (∼0.2 the lysogen level) seen in JL6112 under these growth conditions. To compare the rates of expression with the levels seen in wild-type and λprm240 lysogens, we compared the rate of P_RM⁺ expression at the wild-type lysogen level (panel B), at 2 mM IPTG, with the rate of prm240 expression at the CI level in a λprm240 lysogen. These values were 250 and ∼40 units, respectively; hence, the value for prm240 is 15 to 20% of the value for P_RM⁺, similar to the ratio of observed CI levels in lysogens.